-LRB- CNN -RRB- The attack is harrowing : Al-Shabaab militants raid a quarry in Kenya , separating non-Muslim workers from their Muslim counterparts and executing them .

The brutal act comes just days after the Islamists ambushed a bus and sprayed bullets on those who failed to recite Quran verses .

The attacks reminded the world once again how brazen the group can be .

What does Al-Shabaab want ? Here 's an explainer .

What is Al-Shabaab , and what does it want ?

Al-Shabaab is a Somali group that the United States designated as a foreign terrorist organization in March 2008 . It wants to turn Somalia into a fundamentalist Islamic state , according to the Council on Foreign Relations .

The group has been blamed for attacks in Somalia that have killed international aid workers , journalists , civilian leaders and African Union peacekeepers .

It has a history of striking abroad , too . Before admitting to the Kenya quarry attack , Al-Shabaab was responsible for the July 2010 suicide bombings in Kampala , Uganda , that killed more than 70 people , including a U.S. citizen , who had gathered at different locations to watch the broadcast of the World Cup Final soccer match .

How big is it ?

The total size of Al-Shabaab is not clear .

In 2011 , A U.S. official who declined to be identified because of the sensitivity of the information said Al-Shabaab was estimated to control up to 1,000 fighters .

A U.N. report identified one insurgent leader who is believed to command `` an estimated force of between 200 and 500 fighters , '' most of them Kenyans .

And Al-Shabaab has links to other organizations . In February 2012 , the group 's leader , Ahmed Abdi Godane , and al Qaeda leader Ayman al-Zawahiri released a video announcing the alliance of the two organizations .

How did Al-Shabaab start ?

Decades of weak government amid grinding poverty have long made Somalia a target for radical Islamist groups .

Al-Shabaab 's predecessor was al-Ittihad al-Islami -LRB- AIAI -RRB- , which worked to create an Islamist emirate in Somalia . It was , in part , funded by former al Qaeda leader Osama bin Laden , according to the Council on Foreign Relations .

AIAI , which the U.S. State Department designated as a terrorist group , strengthened after the fall in 1991 of Siad Barre 's military regime and amid the years of lawlessness that ensued .

In 2003 , a rift erupted between IAIA 's old guard -- who were seeking to establish a new political front -- and its younger members , who were seeking to establish fundamental Islamic rule . -LRB- Al-Shabaab means `` the youth . '' -RRB-

That strife led the younger members to ally with a group of Sharia courts -- the Islamic Courts Union -LRB- ICU -RRB- -- that were seeking to impose order over a landscape marked by feuding warlords in the capital city .

Working together , the Islamic Courts Union and Al-Shabaab gainied control of Mogadishu in 2006 . That sparked fears in neighboring Ethiopia that violence would spill over there , according to the Council on Foreign Relations .

Those fears -- combined with a request from Somalia 's transitional government -- led Ethiopian forces to enter Somalia in December 2006 and to remove the ICU from power .

And that move inflamed Al-Shabaab , which attacked Ethiopian forces and gained control of parts of central and southern Somalia , according to a 2011 case study by Rob Wise , who was then with the Counterterrorism Program at the Center for Strategic and International Studies .

What is Al-Shabaab 's relationship with neighboring countries ?

In 2011 , after attacks on tourist destinations in northern Kenya blamed on Al-Shabaab , the Kenyan government ordered a cross-border incursion aimed at creating a security buffer zone in southern Somalia .

Ethiopian troops have also crossed the border and expelled Al-Shabaab from Baidoa , a strategic town midway between the Ethiopian border and Mogadishu .

The group then targeted African Union soldiers and government buildings in the capital in suicide attacks . A suicide bombing in March 2012 killed five people at the presidential palace .

Analysts say tension appears to have been growing within Al-Shabaab between Somalis and foreign fighters , several hundred of whom are thought to have entered Somalia in recent years to join the group .

How does Al-Shabaab recruit ?

The group has a sophisticated public relations arm that includes a Twitter account and video production abilities .

Al-Shabaab has even made a video is as slickly produced as a reality TV show , complete with a hip-hop jihad voice and a startling message :

`` Mortar by mortar , shell by shell , only going to stop when I send them to hell , '' an unidentified voice raps in English .

But Al-Shabaab 's enemies -- and alliances -- can shift .

Al-Amriki , whose real name is Omar Hammami , said in a video posted online last year that he had had a fallout with Al-Shabaab `` regarding matters of the Sharia and matters of strategy '' and feared for his life .

He was reportedly killed in Somalia by Al-Shabaab . CNN was not able to confirm the report .

Finding replacements might not be difficult .

Sheikh Ahmed Matan , a member of Britain 's Somali community , said he knows of hundreds of young Somali men living in the West who returned to Somalia for terrorist training .

How is Al-Shabaab funded ?

The once-ragtag al Qaeda affiliate has grown into an economic powerhouse , raising tens of millions of dollars in cash from schemes that have involved extortion , illegal taxation and other `` fees , '' according to the 2011 United Nations report .

The United States believed then that the group was coordinating with al Qaeda groups in Yemen and might have been plotting attacks in the region and abroad .

In 2011 , it was generating `` between $ 70 million and $ 100 million per year , from duties and fees levied at airports and seaports , taxes on goods and services , taxes in kind on domestic produce , ` jihad contributions , ' checkpoints and various forms of extortion justified in terms of religious obligation , '' according to the report from the U.N. Monitoring Group on Somalia and Eritrea .

How have Somalis been affected ?

In 2011 , the U.N. declared a famine in the southern Somalia regions of Bakool and Lower Shabelle , and Al-Shabaab reversed an earlier pledge to allow aid agencies to provide food in famine-stricken areas .

That year , the U.N. Interagency Group for Child Mortality Estimation said Somalia had the highest mortality rate in the world for children ages 4 and younger .

About 258,000 Somalis died in the famine between October 2010 and April 2012 , and half the victims were younger than age 5 , according to a report from the U.N. Food and Agriculture Organization and the USAID-funded Famine Early Warning Systems Network .

What is the United States doing ?

The United States has supported U.N.-backed African forces fighting Al-Shabaab and strengthened its counterterrorism efforts against the group .

It has also donated millions of dollars in aid .

What is the status of Somalia 's government today ?

In September 2012 , Somali parliament members selected Hassan Sheikh Mohamud as the new president in a vote that marked a milestone for the nation , which had not had a stable central government since Barre 's overthrow 21 years earlier .

But that did n't mean Al-Shabaab was calling it quits . In January 2013 , French forces attempted to rescue a French intelligence commando held hostage in Somalia by the group . The raid left the soldier dead , another soldier missing and 17 Islamist fighters dead .

But there has been political progress in Somalia .

In January 2013 , for the first time in more than two decades , the United States granted official recognition to the Somali government .

CNN 's Tim Lister , Barbara Starr , Paula Newton , David McKenzie and Elise Labott contributed to this report .

@highlight

Al-Shabaab is an al-Qaeda-linked militant group based in Somalia

@highlight

It was once allied with Sharia courts , which tried to impose order on the lawless country

@highlight

The U.S. government designated Al-Shabaab as a foreign terrorist group in 2008

@highlight

Al-Shabaab claimed responsibility for the deadly attack at a Kenyan mall in September 2013